Modes of structural evolution of Abu Gharadig Basin, Western desert of Egypt as deduced from seismic data

Journal of African Earth Sciences, Vol. 9, No. 2, pp. 273-287, 1989 0899-5362/89 $3.00 + 0.00 Printed in Great Britain © 1990 Pergamon Press plc

Modes of structural evolution of Abu Gharadig Basin, Western Desert of Egypt as deduced from seismic data

A. I. BAYOUMI* and H. I. LOTFV**

*Department of Geophysics, Cairo University, Egypt ** Geology Department, E1 Minya University, Egypt

Abstract - The present study, which is mainly devoted to the critical analysis of all available drill hole data and seismic reflection records in the Abu Gharadig Basin area which lies principally to the east of the Qattara Depression in the central portion of the Northern Western Desert of Egypt, indicates that the Abu Gharadig Basin is an elliptical, multicyclic, E-W trending smacturally- controlled basin developed originally as a result of the deep crustal extensional tectonics that affected Northern Egypt during the Mesozoic Era. The basin seems to have been initiated by a localized mantle bulge that led to the restricted deep crustal extension in the area under study during the early phases of the Alpine movements which took place in Late Jurassic. The basin was brought to its active maturation phase during the Cretaceous as a result of the large-scale extensional tectonics along two major bounding E-W trending listric growth faults. Tilting of the basin, took place during the Uppermost Cretaceous/Early Tertiary as a result of the compressive stresses (NW- SE) that were associated with the Laramide phase during these times.

INTRODUCTION

The Cre taceous mar ine rocks of Abu Gharadig Basin, which 11es principally to the e~st of the Qa t ta ra Depress ion be tween la t i tudes 29°00'N - 0 ° 15'N and longi tudes 27 ° 15'E- 9 ° 15'E (Fig. 1), are general ly cons idered one of the mos t impor tan t s ed imen ta ry sequences of the Wes te rn Desert of Egypt in t e rms of pe t ro leum potentials .

The area is general ly character ized by two major surface morphologic features . According to Abu Al Izz (1971), these are (a) the rocky Sou the rn Miocene Pla teau with its no r the rn port ion made up of Middle Miocene-Marmarica "l imestones" (Tm) and its s o u t h e r n por t ions consis t ing of the Lower Miocene Moghra " sands tones and gravels" (Tm), and (b) the Qa t t a ra Depress ion itself which occupies the wes te rn port ion of the bas in area with its floor m a d e up of I~wer Miocene Moghra "clastics" covered by p layas and shallow saline water ponds in its deepest port ions and en- c rus ta t ions , silt, loose s a n d s and Sand d u n e s as well as s a b k h a s in its shal low por t ions (Fig. 2). The Eocene , l imestones , (Te) and the Oligocene "clastics" (To), which are the oldest exposed rock un i t s in the area, are found to crop out in the s o u t h e r n por t ions of the Miocene Pla teau and the Qat ta ra Depression.

SUBSURFACE STRATIGRAPHY AND MORPHOLOGICAL EVOLUTION OF

ABU GHARADIG BASIN

The s t ra t ig raphy of the N o r t h e m Wes te rn Desert was extensively s tudied by so manyworke r s , among those are Said (1962), Norton (1967), Sol iman and Elbadry (1970), Gezeery and Taha (1971), Norton et al ( 1971), Elowi a n d Abdine (1972), Abdine and Deibis (1972), Awad (1984) and Elzarka (1984). However, in a previous s tudy , Bayoumi and Lofty (1985) used the subsu r f ace informat ion ga thered from more t h a n 35 wells (Fig. 6) drilled in the Abu Gharadig Bas in area to t race-out the morphological evolution of the bas in where t hey indicated that:

1) the sed imen ta ry sect ion overlying the Precambr ian b a s e m e n t complex varies consider- ably both in th i ckness and lithology in the Abu Gharadig Bas in area, being th in con t inen ta l a t the n o r t h e m and s o u t h e m port ions, a n d th ick mar ine in the cent ra l portion; (figure 3 is a generalized s t ra t igraphic sect ion showing the major strat i- graphic un i t s in the area a n d thei r character is t ic features);

2) the Abu Gharadig Bas in is a n elliptical multi-cyclic East -West t r end ing bas in , developed wi thin the Nor thern Egypt ian cont inen ta l shelf and being s t ruc tu ra l ly control led from the nor th by

273

274 A.I. BAYOUMI and H.I. LOTFY

a major East-West trending normal fault separating the ma in subs id ing bas inal block from a relatively stable northern platform;

3) the initial phase of formation of Abu Gharadig Bas in appears to have started with the beginning of the first Mesozoic sedimentary cycle. A s low and restricted s o u t h w a r d s transgress ion of the Tethyan Sea, reached Abu Gharadig Bas in area during the Middle Jurass ic , and formed an East-West trending "local embayment . Deposi t ion of the Middle J u r a s s i c Khatatba "sandstones and shales , and Upper Juras s i c Masajid "limestones" followed (Fig. 4A);

:"2~1 l l ~ [ ~ BASIN M E [1 ] T E B R A N E A N $ E A

~0* I@~N N I O ~ ~ ~. BASIN l J

m° ; D E 51 ~ i s ~ D E S E R T

2 o 26, 27" ~ , 2~ r, ~ - 310 3 2 ' 3}" ~4 ~ 35-

Fig. I. Major sedimentary basins in the Western Desert of Egypt (Lotfy, 1984).

4) during the Early Cretaceous more than 1800 metres of s ed iment s represented by the continental Shal tut /Khar i ta "sandstones and shales" and the shal low marine Aptian Alamein "dolomites" were deposi ted in a well p r o n o u n c e d East-West trending elliptical depocenter that had been slowly subs id ing along the major East -west trending fault (F~), The bas in was under shal low m a r i n e c o n d i t i o n s t h r o u g h o u t the Lower Cretaceous establishing the early maturat ion phase

;(;' l~ i L'S/-'*-,4, ¢:x /;, ~ ~o,

5 v l r

~29 o

2; ° oe' 2a" oo'

Fig. 2. Geologic map of the Qattara Depression area (After the Egyptian Geological Survey, 1981). (Te) Eocene, (To) Ollgocene, (Tm) Miocene, (Qd) Quaternary sand dunes,

(Qsb) Quaternary sabkha.

T I M E U N [ T S R O C K U N I T ~. JTIIOLOGY ~uth No~lh

L.) = ~ MIOCENE MOGHRA Contlnen{0 [ ~ l o s h ~ .

0 OLIGOCENE DA B A A :IuvI~'T~II~'It~e" C(3lcarloous

TERTIARY EOCENE LI-J APOLLONIA )pen M~r,ne L,rr, oslon,Ps

PALEOCE NE ( GUI N Ol )

MAESTRICHTIAh KHOMAP- A : t I i I I ~ i l i , ' ~ l z J z 1"{ CAMPANIAH B , --:7-- : : ---z-~,

A

SANTONIAN B U UPPER ~BU C

D ZRE TX~'EOUS TURONIAII ROASH

E i-.-I F

0 CENOMANIAN G

N A L B I A N

O LOWER A P ~ I ~

(J') CRE~CEOUc LLJ RARREMIAN

NEOCOMIAN

E NV[ROt i~.~,~ P I~ ~ "~ h%iC ". :-x K"z*f ~,!rF : PK ~' £1, RF FLECTOF:~

U PPER

JURASSIC "I I D D L E

LOWER . • DEVON1AN LOWER

SILURIAN

F-C~ECA MB R ] A N

[bD ~Q~oa i J

Nerii ic 0 rqamc C, hol W H I g ; t .~eloc{ty

;-' , ' ,' f," Typ+coI Shc l l l c t , H,ql-, ',~'{~c :" H,qh Y~'(OCH ?

' ' ' TO Op~'n Mor ,ne lo~ v e i n : ! r I-, T - .~ ,~ . i in~¢,r ,I,', Nr, i .~t ,,.~ ,-:-~ p-,p--T,-~_.,,...,L~ Limestones W,I h N,O ~', ~H: , , ' v

. - . : _ . . . . . . Sholes A n d Lo~ #e ~c t }

/ Ii / , i { I i HIOP, Veioc , I r - - - - - - - - ~ S a n d s t o n e s Meg,urn Ve~oc,ty t l o p ~ ) ~ - ~

BAHARIYA . - . . . . . Fluvlocnorbne S a n d s L o w Veloc , t , / -T J_

K H A R I T A Cant inental S o ndstorw,~ Low 'veLo~,l~

l l op A~c~m~,~ ] A L A M E I N J/ / ' ~ , I z / / M a r i n e D o l o m i t e s Very HLq.h Veiocff~/~ ....

S H A L T U T - ' ConUnenlol Sandstones Medium w, iocd. y

: ' , ' . . ITop ~4osoj; 'J l m X M A S A j I 0 ~ S h a l l o w M a r i n e

- -' .z ~ l i m e s t o n e s _ _ W E H~jh Ve l~ , ! y

~ H : - - : 2 - : -- ,-~ F o r e _ R e e l Facies MediumVelooty A~ATBA

~ , v A m a . - ~ . \ b M , ' t m ~ . ÷ . k ~ - . - - _ ~ NOT RECOR DE D T ~ Poleozo,c ]

~ _ ~//--- C o n t i n e n l o l To Medlum '¢eloclty GHAZALAT

- " ' * ' ' ~ = - - .%hol low. M ( I r , n e A C A C U c, / , . . __== M~'d ,um VeiOO~y

,.-~ - - Sands lones W*t~

"~ , ' : ; ' - - - - -" M~d,um , ~,~oc,:y G A R G A F . , , / ~ ~ ] g n e o u s IntrLLs,ons

B~_e.~MENT 1 +* .~: ; . . ' Top Ekas~mer~ ]

~ t t t t *

Fig. 3. A generalized column showing major stratigraphic units and reflectors in Abu Gharadig Basin area (Lofty, 19841

Modes of structural evolution of Abu Gharadig Basin,western desert of Egypt as deduced from seismic data. 275

~ - : " " ' - ~ J ~ , ~ ' - 1 1 ,._1 . I . K ~ , , / / ~ . . r , ~ . ~ . ~ - ~ ~ ~

• ~f I | lgg I$OPACN MAP OF THE JURASSIC SECTION ISOPACN MAP OF THE LO~ER CRETACEOUS SECTION

/o

\ -.,.,.

ISOPACH MAP OF The- .~P(~I.ON]A FOR$4~.T1ON ( pALEOCENE. EOCEI~IE)

M

. _

"---90)

ISOPACH MAP OF THE UPPER CRETACEOUS SECTION

Fig. 4. Morphologic evolution ofAbu Gharadig Basin deduced from drill holes.

of the basin (Fig. 4B). During the Upper Cretaceous three thick marine formations of total thickness of more than 2000 metres were deposited in Abu Gharadig Basin as shallow to fluvio-marine Bahariya "sandstones and shales", shallow to open marine Abu Roash "carbonates, shales and sandstones and open marine Khoman "chalky limestones", suggesting a rapidly subsiding basin floor along the controlling fault F I and emphasizing the active matura t ion phase of the basin (Fig. 4 C);

5) tilting of the basin is interpreted to have taken place as a result of the compressional stresses that had affected Northern Egypt during the Uppermost Cretaceous-Early Tertiary. These stresses had ceased the subsidence of the basin along the bounding fault and raised the Syrian Arc System of folds allover Northern Egypt. Deposition of the Paleocene/Eocene Apollonia shallow marine limestones occurred in the newly developed Tertiary Basin to the east of Abu Gharadig (Fig. 4D).

A supporting independent piece of evidence as to the morphological configuration of the basin previously indicated can be readily visualized from the inspection of both the Bouguer and aero- magnetic anomaly maps in the area under s tudy (Figs. 5 A and 5 B) which emphasizes the existence of the major E-W trending fault FI controlling the

basin from the north and running along with the steep gravity gradient separating the East-West trending high anomaly belt occupying the northern platform from the major gravity min imum occupying the main basinal block in the central portion. However, the trace of such a bounding fault (F~) appears to be camouflaged on the magnetic map by the effect of the basaltic and doleritic intrusions recorded in drill holes within the thickness range of the Paleozoic section in the locality where such fault exists and seem to be genetically associated with it.

Also, the inspection of both the gravity and magnetic anomalies in the area under s tudy indicates that another E-W trending fault (F 2) exists as dissected segments, separating the upthrown block of the southern platform from the downthrown block of Abu Gharadig basin, a mat ter which emphasizes tha t the basin is actually a structurally-controlled one, bounded from the north and south by two major normal faults.

As for the basin block itself, it could be consider- ed to be made up of two sub-basins; the first occupying the area around Abu Sennan and the second occupying the central portion of the map area, with the major EI-Kheit gravity hlgh (A), delimiting Abu Gharadig basin from the west, see Fig. 1. Moreover, most of the local gravity and

276 A.I. BAYOUMI and H.I. LOTFY

J / ~ t

!'~ -~ t l " , '

i ~ 0 o Q "' I l ~ K r n 7 7 0 d

l II

~t ~ ~ m l ~ Do i i i _

Fig. 5A Bouguer anomaly map ofAbu Gharadlg Basin a r e a . Contour interval 1 mflllgal (After Oeneral Petroleum Company G,P.C.. 1980),

Fig. 5B Aeromagnetlc map ofAbu Gharadlg basin area. Contour lnter~al of the original data is 5 gammas and the regional component is 317541, (After Aeroservice, 1964).

Modes of structural evolution of Abu Gharadig Basin,western desert of Egypt as deduced from seismic data 277

magnetic anomalies observed in the area are found to have their origin either with the granitic/ granodioriUc basement complex below the basin floor or with the basic and ultrabasic intrusions within the Paleozoic section, such as those en- countered at Sheiba l-X, Sharil I-X and Agnes I-X or both.

SEISMO-STRUCTURAL CONFIGURATION OF THE BASIN

The subsurface s t ructural configuration of Abu G h a r a d i g B a s i n was d e d u c e d f rom the interpretation of more than one-hundred seismic lines shot by three major petroleum companies namely, WEPCO, AMOCO and BRASPETRO and covering more than 30 000 km 2 of Abu Gharadig Basin area. For the sake of brevity, figure 6 illustrates the location of only ten of such seismic lines which show the relationship between theAbu Gharadig basinal block and the bounding northern and southern platforms as well as the major basin-bounding faults. Also, it is indicated here that all our s t ructural conclusions cited in the following section represent, in fact, the resul tant of all interpreted s t ructural elements visualized on the whole net of seismic lines tied together allover the area with 35 drill holes.

A) Structural Configuration based on North- South trending seismic reflection profiles

Six seismic profiles were studied a) line Q-14 (N- S) on the nor thern platform area (Fig. 7A), b) line WFD (N-S) which crosses both the basinal block and the nor thern platform (Fig. 7A), c) lines WEL (NNW-SSE) and WKZ (N-S) within the basinal block (F ig. 713) and d) lines 3BR- 15 (NNW-SSE) and 3BR-8 (NNW-SSE) on the s o u t h e m portion of the map area (Fig.7C). The interpretation of such dip- lines clearly emphasizes the points which follow.

I) The no r them platform is generally characterized by a thin, partially eroded sedimentary sequence as shown in Figure (7A). Among the conspicuous features is the mas te r listric fault previously pointed out as the normal fault Fs affecting mainly the Cretaceous sequence and throwing more than 2000 metres to the south. This growth fault F v which dies-out upwards is generally characterized by a large rollover anticlinal bend within the Cretaceous section indicating the stratal rotation associated with the pulling-apart of crustal blocks on both sides. F~ is important in that it represents the n o r t h e m basin-bounding fault separating the southern downthrown basinal block characterized by a thick marine northerly dipping Cretaceous sequence from the nor thern upthrown stable platform defined by a thin eroded Cretaceous section.

o

NORTHERN

2801 00"

m 2 6 ~

SIgT~t ~

GHARADIG

S141~RI8.!

%-, AGUD_I

AtG.~

,I

'. 3~.1

WEST

0~PL~AG.I~

mvum.i

N,

Fig. 6. Location of the selected seismic lines and drill holes used in the study.

278 A.I. BAYOUW~ and H.I. LOTFY

L I N E TREN0

I1

HE TRENC

I i--

0 u~

I

- I

@

2 ,j < n

LJ ]

_T d

0 <

~E z ~

U_ T I-.--

Z

RFFU ClIO

.~TT TUD

2 o +Z

c_

o o

'RECOGNIZED RE FLE(..TOI:~

T"I, I)a h:~ ~

I+p Ap,~l I , m i

R l , , , "[ur I i ;u ',

h ,p Aft, Roash

FAU L T PATTERN

lh+, f a u l t k l

< <>nspi< ut.tB

m a s t e r ] i~ t r i<

l a H l [ [ fll t}w~

l<! th( ~ ~,I!uth

m(,re t ban

2(XX) met e," s+

RECOGNIZED FA U E T REFLECTORS PAT TERN,, Top ApolIonia The faul t F z is

Base Ter t iary a normal faul t

Top Abu Roaah t h r o w i n g

Top Baharlya southward(4OOm~

+T?p Ma, a a ~ i d ] w i t h a

• Top Peleoz;ic complicated

f a u l t s y s t e m on

Frequent i t s downthrown Unconformities side.

5]- R U CT U R~L CONFIOURATIOI,

} l d c f i r w !h, . i P t t h ( , i n hny~in I~(ulnrllnl~ f ,qt l l /

/ f la t (Or l l ro l~ ; Abll ( ; ! l t ra f lkk : [!;ISJrl. The downthrown

h a s i n a l b l o c k , ~+, t h o ~+mtf l ~ f ( [ 1 ) , i n c l u d e s

thi (k t w ) r t h w l r 6 ,! [~pin£ f'l ~'t acoou~ s e q u e n ( e

! w i l h ;i l a t h e ~,~]! , ,w,I a n t i + l i r l e m a r k i n ~ s t r a t a |

r o t a t i o n , The upthrov+n p l a t f o r m b l o c k , t o t h e

n o r t h of F I , + ' x h i h i t s t h i n p a r t i a l l y e r o d e d

C r e t a c e o u s s e i t i + m ~v i th f r e q u e n t unconformities.

S T R U C T U R A L CONFIGURATION

A thin p a r t l e ] l y eroded sequence within

the northern plat[orm area together with frequent

u n c o n f o r m i t i e s are r e c o g n i z e d .

The southward stepping-out of re f lectora

t o g e t h e r w i t h t h e c o m p l e x f a u l t p a t t e r n [n t h e

s o u t h e r n p o r t i o n c h a r a c t e r i z e the n o r t h e r n

e a s t - w e s t t r e n d i n g b a s i n - b o u n d i n g f a u l t F l which

i s w e l l represented in l ine krFD.

Fig. 7A. Major structural features characterizing both the northern platform and the basinal block along North-South trending seismic lines perpendicular to the regional strike of Abu Gharadig basin.

It could be ment ioned here that the complex pattern of fault ing s y s t e m displayed in the south- e m portion of line Q-14 is referred to the land which co l lapses towards the basin-control l ing fault F~ which lies some distance to the south of this line (Q- 14) as showri in Fig. 6.

2) On the other hand, the Abu Gharadig bas inal block, as displayed on l ines WKX a n d W E L (Fig. 7B), is characterized by a thick northerly dipping, c o n f o r m a b l e , s e d i m e n t a r y , s e c t i o n

(Cretaceous sequence is more than 3 5 0 0 metres thick), cut by normal faulting. Here, the fault pattern is simple. The m o s t important of all is the intra-basin listric normal fault, (F) complicated in places by a set of minor antithetic faults within the Cretaceous formations. However, the uparching recorded on the downthrown side of (F) on line WKZ s e e m s to have been initiated by compress ion s t r e s s e s at d e p t h d u r i n g the U p p e r m o s t Cretaceous times. It s e e m s also poss ible to

Modes of structural evolution of Abu Gharadig Basin,western desert of Egypt as deduced from seismic data 279

U~E TRENt; LOC ~EFLE(:.0N RECOGNIZED! F A U L T I ~K~A'rrl'r,.r,E REFLECTORS P A T T E R N i S [ R t I C I U R A L COI'.IFIGURATION

L3 I O ~ h, I I)ahml } : s i l l I i~- ~l ' lh,, I h i , k t l o l l h ' , v l l 4 d i l ) l , in R C re taceous I M c , , ~ , ' " | r-.J ._~ ~ I l o p ^ l - , I h . H . ' l I I , , ,Re i i i t , :d,~,.~i, m~.¢ t lo . , h a i ; . I , . , i z , ~ s the ^h . ( , I ,a rad l~ hastnnl

G , i

c 3: ~. I o , ~- ~ ~ - T-i. ^ i . . . ~. a l . s u ~ . . r d .~d

I J c I . . ....... . , 'mr, . . . . . t | the4 '¢ | IJJ ~- "1"- ~>" i C , , i ) l o r m a b l e , , [ ~ ~ -~ . . . . . . , ~ .

, i i

I . . . . . . . . . " i 1 / i "®

I m:

/

I / i ~ ~ , ~ , , . ~

Fig. 7B. Major structural features characterizing the basinal block along North-South trending seismic lines perpendicular to the regional strike ofAbu Gharadig Basin.

consider such a fault (F) as being the southern basin-bounding fault during the Lower Cretaceous since it separates a thick Lower Cretaceous sequence on its downthrown side to the north from a relatively thin sequence in its upthrown side as shown in line WEL.

3) The sou thern platform as it is displayed on lines 3BR-8 and 3BR-15 (Fig. 7C) is characterized by northward dipping strata accompanied by northward thickening of the sedimentary section. Few of these normal faults are extended upwards to affect the Tertiary sequence whereas m a n y of them die upward at the Base Tertiary level. The fault F2, (indicated on the gravity and magnetic map as dissected segments), is the most conspicuous of all faults displayed on both lines. It is a large listric normal growth fault down to the north, associated in places with minor antithetic faults

(marking the periodic re-activation of sliding movement along it during the Cretaceous times); in fact (F 2) generally represents the maJ or southern basin-bounding fault, separating the subsiding downthrown basinal block to the north from the relatively stable upthrown southern platform block. The Top Abu Roash is t runcated along the Base Tertiary in the s o u t h e m portions of both seismic lines (3BR-15 and 3BR-8).

B) Structural Configuration Parallel to the Regional Strike of the Basin (East-West trending profiles)

Here four seismic lines were interpreted, a) line 26 (E-W) which lies along the nor thern platform area, b) line WKH (WNW-ESE) and line WKX (E-W) within the expected basin boundaries and c) line 3BR-11 (ENE-WSW) along the sou the m platform.

28() A.I. BAYOUMI and H.I. L()'r~

,

a: ~5 m

o

c ~

® o

I~COGNIZED ~FLEC TORS

- Top A p o l l o n i ~

- I ~ s e T e r t i a r y ]

- Top Abu E o a s ~ " ~

- Top I ~ h a r l v ~ I - Top AI~r~eln

- I~Isemen t

* F r e q u e n t Unconforml t Ie~

FAULT PAT T ERN

C e . e r e J I y

Comp I ox w i t h

r l u ~ r o i ls f a u l t s

!F 2 |e ~ M J o t

no rma l I I s t r i c

fro, I t I h r o w l n R

n o r t h w R r d ,

i

I

$ T R U C T U RA L CONFIGURATION i

Fa. l l F 1 d ~ ' [ i n r ' ~ lhr~ ~ o l l t h ~ r n h ~ s l n - h o t l n d | n ! l l

ICeu I t . T(~ I h r n~,r ! h ex l,ql s thP downt hrowI hill

s , . h s i d t . R h r i ~ i , m I blc~rk o f Ahl l P, hAr~dl IR ( t h i r k

Ur e l ac~-nll~ s * c t t~m. . h i I t h e ~ h f tqu l t n 41hd

. . F,~rw~l, I ~ , ~. I ul" i o , ~ h t p.~ } . To I he Rm| th t he

q ) l h r ~ w I . R ~ l n h l ~ p l e l [ o l ' l b l o c k @xlst.q ( t h i n

i n r ! l a l I T e r ~ l e d ~-qt l~.nce w i t h . n c c m [ o r i i t l e ~ i ) .

Fig. 7C. Major structural features characterizing the southern platform along North-South trending selsmlc lines perpendicular to the regional strike ofAbu Gharadig Basin.

I) The n o r t h e m platform, a s it is d isplayed on line 26 (Fig. 8A), s e e m s to have b e e n character ized b y a th in part ial ly eroded sed imen ta ry s equence striking Eas t -Wes t cu t b y n u m e r o u s normal faul ts (with or wi thou t ant i thet ic faults) affecting main ly the Cre taceous and older s t ra ta . It is indicated tha t the Top Abu Roash could only be t raced ou t in the wes t e rn por t ion of the sect ion due to erosion fur ther eas t in the u p t h r o w n b lock of the normal fault (Fu). This is conf i rmed by da ta obta ined from Sheiba-lX well. At th is well location, erosion or non-depos i t ion of the Apt ian Alamein "dolomites" resu l t s in the Top Masaj id (Top Jurass ic ) to be the f i rs t r ecogn ized re f lec tor be low the Ear ly C e n o m a n i a n Bahar iya reflector.

2) The Abu Gharadig bas ina l b lock itself, as d isplayed on lines WKH and WKX (Fig. 8B) that lie within the expected b a s i n boundar ies , is general ly character ized by an Eas t -West striking th ick

conformable sed imen ta ry sect ion compl ica ted by normal faulting affecting main ly the Cre taceous and older rocks. Both lines also d isp lay tha t the Base Tert iary as well as o ther older hor izons lie at relatively deeper levels than those recorded on lines 26 and 3BR- i I {Fig. 811) wi thou t any m a r k e d unconformity.

3) The s o u t h e r n platibrm, on the o ther hand, as it is d isplayed on line 3B3R- 11 (Fig. 8A), like line 26, is general ly charac ter ized by Eas t -West s t r ik ing s t r a t a wi th t h in par t ia l ly e roded Cre taceous sequence cu t b y n u m e r o u s normal faul ts assoc ia ted with several ant i thet ic faults . The unconformi ty be t ween the Base Tert iary and the Top Abu Roash is charac ter ized by the part ial or complete eros ion for non-depos i t ion of the Campan ian K h o m a n ,chalk, as is shown by the t runca t ion of the Top Abu Roash. The Early Tert iary sec t ion ontaps a t the wes t e rn end of line 3BR- I I.

Modes of structural evolution of Abu Gharadig Basin,western desert of Egypt as deduced from seismic dam 281

UP N

tREND UOCM~

e

,iT i ITUDE

_c .Z"

!

RECOGNIZED REF LECYORS

- lop lbu lonlh

-_Lop..l/N~ , a

- T.17-F. l lo~oi~

• Frequent IksrMf~r l l~ t ld

FAU LT PAT T ERN

~ l p l l c | l l d

mtiern o[ nnrml[

[aSit S y i t ~ .

eel ii _ _ .

r~ I RUCT" U RA L CON F IGU R^T IOk l

"l'ypicll E~abl~ plstlorm fSatVi~¢ d i i l i l y i n l l

~ l s t - i o t i f l ~ r - / y i l , ¢ ~{ ra i# v i t h t f i i . l l i r l i / l l / #.i~llt~ reuited C i l l i c . o u s s e c J : h , . ~1; ~m.Jl a5 ~re~melt~

Ilnconf0rmitips.

°

- n

t I I

4 1 z LU

L ~

Z

~ E

w I

~FtJRV0m AT]'BUm[

q~

._~ I

r~

-_h- T-,.. v - Top ~ loush

Top Ileher lyu

Top Alkal i n

• Prequeat Ilvuc~ forml t i •

7a811 vort tool d t Iplacement I i81o~ 8 n m r o u 8

feu l tn essocisted

v i th minor

ant i thet ic remits,

SOUTHERN

S T R U C T U R A L CONFIGURATION

TTpical stllble p181~form features showJnR f [& t . | y in l l

! # r l y }w)t'tzemtlJl straits ~no r o t l t i o n ) rhsrsc ior |zed by

I:hJn p 0 r t i a l l y eroded fllulted Crelireou,q R~quPnt'p with

~eq.ent UneOnrormt ¢iffs.

Fig. 8A. Major structural features characterizing the stable platform areas along East-West trending seismic lines parallel to the regional strike of the basin.

C) Depth S e c t i o n s It is indicated here that most of the major

structural elements of Abu Gharadig Basin that are clearly outlined on the seismic lines, particularly those trending North-South, can be clearly observed on the two structural cross sections (Fig. 9) which have been prepared using the constructed set of average velocity contour maps shown in figure 10, and by making use of data obtained from more than 35 drill holes (Fig. 6). A£S 9:2-F

MAJOR STRUCTURAL/MORPHOLOGIC M ~ OF ABU GHARADIG BASIN

According toAnderson e t a/. (1983), structurally- controlled sedimentary basins evolve generally either as: a) sagged faulted basins, associated with narrow zones of deep plastic extensions located directly beneath the deepest part of the sag and separated from one another by 20 to 30 km, b) large homoclinally rotated basins associated with zones of deep plastic extension that are laterally

282 A.I. BAYOUMI and H.I. Lo'n-'Y

LINE }TREND LOCAIION REFLECIION AIIIIU~E

I

" 0

0 e

~q X < (

LU

t ~ UJ Z

F -

®

• ~ . _~

{=x 0 T ~ --~

~ 0 C~ m 0 <[ --..

RECOGNIZ ED RE FLECT(~c~,

- Top Dabaa

- Top A p o l l o n i a

B a s e ~ e r t i a r y

- Top Abu Roash

- ' F o p F ~ a h a r i w

-- rI'Op glamein

G e n e r a l Iy Con f o r m a b l e .

FAULT PA T T ~RN

G e n e r a l l y simple

w i t h smal 1

f a u l t s t h a t

h a v e l i t t I~,

i n f l u e n c e in the

b a s i n c o n t r o l .

ST RU CI U RAL I

CON FIGURATION

' l y p ] ( a l hasina) b l o c k f e a t u r e s showin R

e a s t - w e s t s t r i k l n ~ f } a t - l v i n R h o r i z o n t a l s t r a t a

with t h i c k (if el a c e o u ~ sF,(l uellc e a s w e ] ] a s

( : o r l f ( ) r m a b l e r e h t i o n s h i p .

Fig. 8B. Major structural features characterizing the basinal black along East-West trending seismic lines parallel to the regional strike ofAbu Gharadig Basin.

displaced from the surface trace of the master listric faults by distance depending on the radius of curvature of the major controlling fault, or c) complex basins which are assemblages of complexely deformed sub-bas ins (mosaics) associated with sharply curved shallow listric faults merging downwards into a large planar fault representing a detachment surface as shown in figure 1 1.

As far as the nature and geometry of the basin- controlling faults are concemed, they are generally

either planar (steep or gentle) or listric flatened-out with depth. Most of these master faults are generally associated either with a conspicuous pattern of down bending or flexing of the basin-fill sediments against the fault plane (the so-called rollover anticlinal bends) and/or antithetic faulting representing the gravitational collapses of the platform inside the basin as it evolves to accomodate the space from pull-apart of crustal blocks associated with the deep plastic extensional deformation as shown in figure 12.

Modes of structural evolution of Abu Gharadig Basin,western desert of Egypt as deduced from seismic data, 283

NORTH SOUI~

L ine 3 BR_IS L ine WF D L ine WKZ L ~ ,I

W O.]L|

®

Pt |RMeosolc I

OASIC llJA~I4FM! CONI~I°I. E It

i HORTH .~OtrTH

Line Off, ~ I L i . e W E L _ i L i . e 3 B I ] . 8 •

~ _ _ ~ ~ r ~ ~ - - J . . . . " --=

~ ¢1 N r e

W' bJ • ,~, " IC

i

-" : | PLA IF?RMI

Fig. 9. Structural configuration ofAbu Gharadlg Basin along the North-South trend, interpreted from seismic data, using average velocity contour maps (Fig. I0). and drilling data (vertical exaggeration 10X).

In view of the above-mentioned considerations, it is interpreted here that:

I) The Abu Gharadig Basin is a homoclinally rotated basin developed within the nor thern portion of the African plate and controlled by two major East-West trending listric growth faults (F~) and (F 2) with F~ being the nor thern basin bounding fault separating the downthrown basinal block from the nor thern stable platform and F 2 the southern bas in bounding fault separating the subsiding basinal block from the southern relatively stable platform.

2) The Abu Gharadig basinal block, which occupies the central portion of the area under

study, is characterized by a thick marine Cretaceous sequence dipping nor thwards and including within its nor thern portion, a large tensional rollover anticlinal flexure. Within the southern portion of the basinal block, the basin floor rises gently southwards toward the s o u t h e m platform. The Cretaceous sediments are fractured by several minor anti thetic faults on the downthrown side of F 2 marking the periodic subsidence of the basinal block with respect to the southern platform along F 2 as a response of the cont inuous sliding of the basin along F~ to the north (since Jurass ic times). Within the basinal block, in general , the Upper Cre taceous

284 A.I. BAYOUMI and H.I. LOTFV

__~ ~o--~i Z •

/ / / /

]TOP ALAMEIN ~ . ~ " \

m,

ze~ z ~

2%w i

: ~ 7¢,0¢T

© . . . . . . . . . . //A

Fig. I0. Average velocity contour maps on the major reflectors within Abu Gharadig Basin area (Lofty, 1984).

Q:

Fig. 1 1.

l ~ d e (A)J

~ e d - faulted has, 4

' r

lOKm

20Kin

3C ~-, .

E e s t r i c t e d p l a s t i c c r u s t a l

extension due to a local ized

annt le bulge in the quasi-

continuous zone of p las t i c

ex tens ion r e s u l t i n g ane2astic

st retch ing and th tnn in B

of the crus ta l slob. [Stt~m't

1971, and Wallace, 1979]. i

Sassed bas in above the

d u c t i l e s t r e t c h i n 8 zone.

Ares w i l l be aetmmically

8 c t l v e with p e r i o d i c l u r f s c e

ruptures serked by ant i the t~

f a u l t i n g with r e s c t i v s t l o n

of s l i d i n g along the Ims te r

f a u l t end deepening of

the basin.

l~oee c B )J

r , i

Basin

" ~ l . . . . . . . . C4 Moho

Deep plastic-extension

aSSOCiated with decouplin 8

d u c t i l e t r a n s i t i o n zones

msit ioned some distance

from the basin axis in

downdip of the =mjor l l s t r l c

f a u l t .

~on,p,lex toas*n ( Mosaic ),i

• 1(~ m :

Mcl'~o ~JO~ rr

g r e a t Regional crustal extensions at

distances away from the basins axes.

lmrBe k e m c l i n t l ro ta t i on

of bas in f l o o r b locks a long

the ImJor l i n t r i c f a u l t .

A s e t of t h i n - s k i n n e d e x t e n s i o n a l b a s i n s

s u b s i d i n g a long a se t of master l i s t r i c

f a u l t s s e r 8 t n 8 downward i n t o a p l snn r

d e t n c h l e n t f a u l t which e i t h e r :

A- Projects to decoup l in 8 zones r ep r e sen t i ng

duc t i l e t r a n s i t i o n zones in the crust .

[Eaton, Ig79 and l e Pichon and S ibue t ,

198] ]. or

B- Extends as a p lanar f a u l t a long which

i rXs~d c r u l t a l ,slabsi dtverJei~i,, [ ~ h J c k e , 198tj

Modes of s t ructural evolution of sedimentary basins (Modified after Anderson et aL, 1983).


Case A Themaster listric fault is characterized by minor

ontithetic faults markircj gravilational cOItapses.

Case B The master listric fault is characteriz~ i~ rollover

anticline ma~ir'~ tl'~ stratal rotation

F

. . . . . . .

f

Fig. 12. Cross sectional sketches showing strata] response to accomodate the void space arised from pulling-aparat of blocks and the subsequent subsidence of the basinal block along the listrlc fault (Modified after Anderson et al., 1983).

format ions are frequently folded, having the general t rend pa t t e rn of the Syrian Arc sys tem of folds.

3) To the nor th of the nor the rn bas in bound ing fault F I the no r the rn platform includes generally th in partially eroded cont inenta l Cretaceous sedi- men t a ry sequence, with f requent unconformit ies , (the C a m p a n i a n K h o m a n "chalk" and the Apt ian Alamein ~dolomites, are thin, partially eroded a n d / or non-deposi ted) and the sed imentary uni t s generally dip nor thwards towards Mat ruh Basin.

4) To the sou th of the sou the rn bas in -bounding fault F 2, the s o u t h e m platform also shows, th in partially eroded continental sequence dipping gently nor thwards toward the Abu Gharadig depocenter.

MODES OF STRUCTURAL EVOLUTION OF THE BASIN

The s t ruc tura l evolution ofAbu Gharadig Basin, within the n o r t h e m port ion of the African plate, which developed as a resul t of the deep and complex extensional tectonics prevailing across Northern Egypt dur ing the t ime span of the Alpine Orogeny tha t had resul ted in the opening and t h e n the subsequen t closing of the Te thyan Sea along the no r the rn marg in of the African plate, is interpreted in figure 13 where

1) the left lateral drift of the African plate relative to the European one, which h a p p e n e d in the Middle Ju rass i c and dur ing the early phases of the Alpine Orogeny as a resul t of the spreading of the central port ion of the Atlantic Ocean, was according to Wood (1984), generally responsible for the opening of the Tethys and for the init iat ion of Abu Gharadig bas in th rough a localized mant le bulge in the Qattara Depress ion area, th is indicated by the late Ju rass ic basalt ic in t rus ions prevailing Northern Egypt in this par t icular locality.

(2) During the Cretaceous, the tensile s t resses prevailing Northern Egypt dur ing the Alpine movement , as a resul t of s u c h sinistral drift of Africa, which reached its acme dur ing the Upper Cretaceous, m u s t have been so active to the degree tha t the floor of the Abu Gharadig Basin s tar ted a regional homocl lnal rota t ion along the major East- West t rending listric fault (F~), leading to the deepening and broadening of the bas in and bring- ing it up to a wel l -pronounced ma tu ra t i on phase especially dur ing the Turonian.

(3) From Turon ian t imes onward, the con t inued opening of the Central Atlantic Ocean was eclipsed by a more rapid opening of the North Atlantic Ocean result ing in a reversal of the relative mot ion between North Africa and Europe to dextral and leading to the initial stage of closing the Tethys (Wood, 1984). This Pos t -Turonian right-lateral t r a n s c u r r e n t movemen t along the n o r t h e m bounda ry of the African Plate resul ted in the compressive s t resses tha t were responsible for the death and decl inat ion ofAbu Gharadig as a bas in and beginning a tilting phase associated with the Laramide phase of the Alpine Orogeny. The elevation of the Syrian Arc Sys tem of folds over Northern Egypt including Abu Gharadig Basin itself is mos t probably related to such Turon ian dextral drift of Africa relative to Europe.

CONCLUSIONS

II Critical analysis of all available exploratory data, including drill hole, gravity magnet ic and seismic reflection da ta indicates tha t the Abu Gharadig Basin, which ha s proven to conta in large reserves of hydrocarbons , within its th ick mar ine Cretaceous sequence, is an extensional, elliptical, multicyclic, E-W t rending structural ly-control led sed imentary bas in developed within the nor the rn port ion of the African plate as a consequence of the left-lateral drift of N o r t h e m Africa relative to Europe which s t a r t ed in Middle J u r a s s i c times.

21 The Abu Gharadig Basin exhibits on typical bas in /p la t fo rm s t ruc ture a subsiding bas ina lb lock i m p l y i n g e n o r m o u s l y t h i c k c o n f o r m a b l e Cretaceous mar ine sequence b o u n d e d nor thwards

286 A.I. BAYOUMI and H.I. LoTvY

AGE

MIOCENE

EOCENE

J URASS IC

TECTONIC II2NelRY I N T PREVAILED STRESSES STRUCTU~,AL RESPONRE~.

was ,~ound end e f f ec t i ve in the Gull r of Suez reRi-n ] RIFTING where s Miocene besin ~ v e d malnl b r i f t i -

(.D 0

rr"

I (_9

~ ~]-evetion nf the Syrian arc

stresses prevailed dt, r i n l ~ W l t h s general re e . . . . . . . . . . . . . i the ~ plm.qe of the I| [rei~vpnntl~,n o[ mnvement~

klptne'OroReny I I I ~j,,,g old f,~,,,,s.

A NW-SSE tenJJ~e s t r e s s ~ L e r g e scale deep crnstg] that prevai led the ~ e~tenslon associated with North Western Desert ]srge homocI|n~| ro ta t ion during the CretaCeous aJon 8 the major ] is tr lc [aults sad reached i t s scene tn ]ceding to the deepening and

the TuronJan broadening of Ahu GharadlR basin

localized dolerltlc ~ x t e n s i o n accompanied by Ind bsssJtlc mgntle h~illlOw mild suh.qidence along m|ee intruded the ~he old major east-~e~t hit tsru depressinn arPa f r 8ctures

TRIASSIC

LATE

EARLY

. rEpeJrosenic In nature ceuswd a genPra[ up l i [L and 1 H E R C Y N I J ~ J ~ J i ° ~ - ~ - - ' ] l l ~ w l d e s p r e a d leRression eJJ ov~r Ihe North Wester,, [~czrr,

J l .... itin, , .... depo~i, i ..... , P.rmi ..... d Tri .... i. J

Fig. 13. Speculations on the mode of structural evolution of Abu Gharadig Basin.

and s o u t h w a r d s by two relatively s table p la t forms showing th in part ial ly e roded Cre taceous success - ion. Two ma jo r listric growth faul ts (F I and F 2) b o u n d the ma in bas ina l b lock and control its s u b s i d e n c e with respec t to the nor the rn and s o u t h e r n platforms. Star t ing from the s o u t h e r n platform and going bas inward , the sed imenta ry s equence is dipping gent ly no r thwards and also thickening gradual ly bas inward towards the basin- bound ing fault F i.

3) Initiation of the Abu Gharadig Bas in s tar ted dur ing the Late J u r a s s i c a s a resul t of the in t rus ion of a deep-sea ted basa l t ic and doleritic mant le bulge, related to the early phase of the Alpine Orogeny. This resu l ted in a res t r ic ted c rus ta l extension, a ccompan ied by a slow mild subs idence of the bas ina l block, along a n Eas t -Wes t t rending f racture (F:) in the Nor thern Wes te rn Deser t of Egypt.

4) Major s t ruc tu r ing in the bas in s ta r ted in the Lower Cre t aceous and con t inued th roughou t the Cre taceous epoch a s a resu l t of the regional tensile s t r e s ses wh ich had affected the no r the rn port ion of the African plate dur ing tha t t ime and reached its acme in the Turonian. A large scale c rus ta l exten- s ion occur red assoc ia ted with regional homocl inal rotat ion of the sliding bas ina l b lock of Abu Gharadig along the no r the rn bas in bound ing listric fault (FI). This lead to the deepening and b roaden ing of the basin.

5) The Post Turon ian dextral drift of Africa relative to Europe c rea ted compress ive s t r e s se s in the N o r t h e m port ion of the African Plate and resul ted in uplift of the Abu Gharadig Bas in dur ing the U p p e r m o s t C r e t a c e o u s / E a r l y Tertiary.

Aeknmuledj/ements - The authors wish to thank the authorities of the Egyptian General Petroleum Corporation (E,G.P.C.) for suppl-~ng the exploratory data used to conduct the present study which are confined to uninterpreted one hundred seismic lines, gravity, magnetic and weU log data obtained from more than 35 drill holes; interpretation of such exploratory data in terms of subsurface geologic conditions was the main responsibility of the authors.

REIr~RENCF_~

Abdine, A.S. and Deibis, S. 1972. Lower Cretaceous Apt|an sediments and their oil prospects in the Northern Western Desert, Egypt. Eighth Arab Petroleum Congress, Algeria.

AbuAl Izz, M.S. 1971. Landforms of Egypt, translated by Y.A. Fayld, the American University in Cairo Press, Cairo, Egypt.

Anderson, R.E. et al. 1983. ImplicaUons of selected subsurface data on the structural form and evolution of some basins in the northern Basin and Range province, Nevada and Utah: Geological Society of Amerlca Bull., 94, 1055-1072.


Awad, G.M. 1984. Habitat ofoil in Abu Gharadig and Fayum Basins, Western Desert, Egypt, ~ Bu//., 88, No. 5, 564-573.

Eaton, G.P. 1979. Regional geophysics, Cenozoic and geologic resources of the Basin and Range province and adjoining regions. In:. Basin and Range Symlx~lum Rocky Mountain Association of Geologists and Utah Geological Association. (Edited by Newman, G.W., and Geode, H.D.), 11-39 p..

Elowi M. and Abdine, S. 1972. Rock units correlation chart of the Northern Western Desert, Egypt, Wepco Petroleum Company.

El Zarka, M.H.1984. The application of facies para- meters and mapping techniques to exploration of the subsurface Lower Cretaceous of the Qattara Depression, Western Desert, Egypt, Journal of Petroleum Geology 7, 277-302.

Gezeery, N. and Taha, I. 1971. Contribution to the stratigraphy, tectonics and oil shows in Abu Gharadig Basin, Western Desert, Ninth Annual Meeting Geological Society of Egypt, Cairo.

Le Pichon, X., and Sibuet, J. 1981. Passive margine: A model of formation: Journal of Geophysical Research, 88, 3708-3720.

Lofty, H.I. 1984. A geophysical study on the Qattara Depression Area, Western Desert, Egypt, M.Sc. thesis, El Minya University, Egypt.

Norton, P. 1967. Rock stratigraphic nomenclature of the Western Desert, Exploration Department report No. 4, Pan American Oil Company, Cairo, Egypt.

Norton, P.,Hagras, M.andAbdallah, A.M. 1971. Progress report on rock stratlgraphic units in the central part of the Western Desert. Ninth Annual meeting, Geological Society of Egypt, Cairo.

Said, R. 1962. The Geology of Egypt, Elsevier Publishing Company, Amsterdam.

Soliman, S.M. and Elbadry, 0. 1970. Nature of Cretaceous Sedimentation, Western Desert, EgyptAA/~ Bul/etln, 54, 12, 2340-2370.

Stewart, I.H. 197 I. Basin and Range structure: A system of horsts and grabens produced by deep-seated extension. Geological Society of America Bulletin, 82, 1019-1044.

Wallace, R.E. 1979. Earthquakes and the pre-fractured state of the western part of the North America continents Proceeding of the International Research Conference of Intra-cont inental Ear thquakes , September 17-21, 1979, Orhld, Yugoslavia, 69- 81 p.

Wemicke, B. 1981. Low-angle normal faults in the Basin and Range province: Nappe tectonics in an extending orogen. Nature, 291, 645-648.

Wood, D. 1984. The Tectonic setting and structural evolutions of Abu Gharadig Basin, Western Desert of Egypt, Seventh Exploration Seminar, Cairo.

Documents

Modes of structural evolution of Abu Gharadig Basin, Western desert of Egypt as deduced from seismic data